Crustal structure in the southern Andes, adjacent foreland, and Atlantic passive margin delineated by satellite gravimetric models

CHAPTER

Crustal structure in the southern Andes, adjacent foreland, and Atlantic passive margin delineated by satellite gravimetric models

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Mario Gimenez⁎,†, Agustina Pesce⁎,†, Stefanie Pechuan⁎,†, María Alejandra Arecco‡, Santiago R. Soler⁎,†, Sebastián Correa Otto⁎,†, Federico Lince Klinger⁎,†, Orlando Álvarez⁎,†, Andrés Folguera§ Seismological Geophysical Institute Ing. Volponi (IGSV), FCEFyN, National University of San Juan, San Juan, Argentina⁎ National Scientific and Technical Research Council (CONICET), Buenos Aires, Argentina† Instituto de Geodesia y Geofísica Aplicadas "Ing. E. Baglietto", FI, Universidad de Buenos Aires, Buenos Aires, Argentina‡ Department of Geological Sciences, National Scientific and Technical Research Council (CONICET), IDEAN— Institute of Andean Studies "Don Pablo Groeber", FCEN, University of Buenos Aires, Buenos Aires, Argentina§

1 ­Introduction In the last years, several works have characterized and discussed the crustal structure over the Southern Central and Patagonian Andes and adjacent foreland region by using geological and geophysical tools (Bassin et al., 2000; Bastow et al., 2008; Meijde et al., 2013; Park et al., 2008; Tassara, 2005; Tassara et al., 2007; Sacek and Ussami, 2009). These works have validated, to a certain degree, previous interpretations on the crustal structure that supports the Central and Patagonian Andes based on the description of ophiolitic sections, allochthonous fauna, exposed arc roots defining batholitic belts, regional shear zones, metamorphic belts, etc. (Heredia et al., 2018; Ramos, 1984; Ramos et al., 2011; Rapela et al., 2011). The resulting picture shows to be highly heterogeneous, depending on exhumation degree achieved during Andean mountain uplift: While the Chilean-Pampean flat subduction zone between 27 and 33°S has exhumed a complex crustal structure characterized by ophiolitic belts and ancient arc and metamorphic belts that allows analyzing a rich mosaic of Pampean-Panafrican and Famatinian accretions, other sectors show a much younger coverage precluding this kind of analyses, such as the Patagonian region. In this work, we evaluate the different proposed crustal structures by using Earth gravity field models, describing the main density heterogeneities from the Pacific trench to the offshore basins resting on the Atlantic passive margin (Fig. 1). In particular, we used the Earth gravity field model EIGEN-6C4 (Förste et al., 2014), a high-resolution model, which integrates satellite data such as GOCE (The Gravity field and steady-state Ocean Circulation Explorer), GRACE (the Gravity Recovery and Climate Experiment) and LAGEOS (Laser Geodynamics Satellite), and terrestrial data from EGM2008 model (Pavlis et al., 2008, 2012). The EIGEN-6C4 model Andean Tectonics. https://doi.org/10.1016/B978-0-12-816009-1.00002-2 © 2019 Elsevier Inc. All rights reserved.

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FIG. 1 Main geological structures, morphostructural domains, different proposed terranes and tectonic lineaments mapped over a digital elevation model, SRTM30 (Becker et al., 2009). SP, Pampean ranges (Pampean broken foreland zone); NPM, North Patagonian Massif; DM, Deseado Massif; JFR, Juan Fernandez Ridge; MFZ, Mocha Fz.; VFZ, Valdivia Fz.; MFFS, Magallanes Fagnano fault zone; RPC, Rio de La Plata Craton; F, Famatina system; P, Pampia terrane; CY, Cuyania terrane; CH, Chilenia terrane. Earthquakes locations with M > 4.5 (NEIC) and quaternary arc volcanoes are shown in black-white circles and red triangles, respectively. The Chilean-Pampean flat slab (between 27° and 33°S) coincides with a gap in arc activity and a higher seismicity rate. Section A-B is used for development of a forward gravimetric model depicted in Fig. 4. The displayed crustal structures were extracted from previous models: (1) Alvarez et al. (2014), (2) Alvarez et al. (2012), (3) Ramos (1988), (4) Chernicoff and Zapettini (2004), (5), (6), and (7) Arecco et al. (2016).

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is developed up to degree/order 2190 with a spatial resolution of λ/2 = 9 km. We used the satellite only GOCE model GO_CONS_GCF_2_DIR_R5 (Bruinsma et al., 2013), a full combination of GOCE-SGG (Satellite Gravity Gradiometer), GOCE-SST (Satellite-to-Satellite Tracking), GRACE (Gravity Recovery and Climatic Experiment), and LAGEOS (Laser Geodynamics Satellite). The last model, developed up to degree/order N = 300 (λ/2 = 66 km), was used to interpret the lithospheric structure related to density variations in the Southern Central Andes and Patagonia. The last model has homogeneous precision and regional coverage being useful for regional studies and to interpret the gravity-derived signals at mediumto-high wavelengths (Alvarez et al., 2012, 2014, 2015; Braitenberg et al., 2011, Braitenberg, 2015; Li et al., 2013; Mariani et al., 2013). To study the Atlantic passive margin region, we used data from COPLA (National Commission on the Outer Limit of the Continental Shelf). These data consist of bathymetric, gravimetric, magnetic, and multichannel seismic data grids of 23,000 km, acquired by the Federal Institute for Geosciences and Natural Resources (Bundesanstalt fur Geowissenschaften und Rohstoffe, BGR, Germany) and 6900 km of multichannel seismic data from COPLA (Arecco et al., 2016).

2 ­Methodology The Earth gravity field model is obtained from the observed potential. Then, the disturbing potential (T) is calculated by subtracting the potential field of the reference ellipsoid (WGS84) (Janak and Sprlak, 2006). The disturbing potential allows calculating different derived quantities, related to the crustal density heterogeneities.

2.1 ­The topography corrected vertical gravity gradient (Tzz) The gravity gradient tensor (Marussi tensor) is obtained as the second derivatives of the disturbing potential T (e.g., Hofmann-Wellenhof and Moritz, 2006) and is composed of five independent elements. The Marussi tensor components M = Tij can be expressed and solved numerically in a spherical coordinate system (Rummel et al., 2011; Tscherning, 1976). The vertical gravity (Tzz) gradient is the second radial derivative of the disturbance potential T. Tzz =

∂ 2T ∂r 2

(1)

The vertical gravity gradient highlights superficial density anomalies and allows delineating the location of an anomalous mass with better detail and accuracy than the gravity anomaly itself (Braitenberg et al., 2011). Since the Tzz is a derivative of gravity, the spectral power of gravity gradient signals is pushed to higher frequencies, resulting in a signal more focalized to the source than the gravity anomaly (Li, 2001). Therefore, Tzz is better for detection of the edge of geological structures and to distinguish the signal due to a smaller superficial density variation from an extensive deeper mass, being the gravity anomaly more sensitive to regional signals and deeper sources (Alvarez et al., 2012). Relative denser bodies are related to a positive gradient value while a negative gradient is related to less dense bodies. Abrupt changes may indicate a high-density contrast between two different lithologies. For calculation of the vertical gravity gradient (Janak and Sprlak, 2006), we used the model EIGEN-6C4 (Förste et al., 2014) up to its maximum degree/order = 2190 (Fig. 2), and the model GOCE GO_CONS_GCF_2_DIR_R5 (Bruinsma et al., 2013), up to degree/order = 300 (Fig. 3).

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Chapter 20  Crustal structure delineated by satellite gravimetric models

FIG. 2 Vertical gravity gradient from EIGEN-6C4 model up to degree/order N = 2190 (Förste et al., 2014) corrected by the topographic effect. A-B is the location of the cross-section of the gravimetric model shown in Fig. 4. References are in foot note of Fig. 1.

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FIG. 3 (A) Vertical gravity gradient from GOCE GO_CONS_GCF_2_DIR_R5 up to degree/order N = 300 corrected by the topographic effect. (B) Detail of the North Patagonian Massif area.

For an approximate calculation of the effect of the topographical masses above the ellipsoid, we used the global relief model ETOPO1 (Amante and Eakins, 2008) and a mass density distribution hypothesis. We utilized standard densities of 2670 kg m−3 for continental crust and a density of 1030 kg m−3 for the sea water. The topographic effect was calculated at a height of 7000 m to ensure that all values are above the topography and was made in a spherical coordinate system. The topographic elements were approximated using spherical prisms or Tesseroids (Alvarez et al., 2013; Uieda et al., 2010), taking into account the Earth curvature in order to avoid the error introduced when using a planar approximation (Bouman et al., 2013; Grombein et al., 2013). All calculations were carried out with respect to the system WGS84 on a regular grid of 0.05° grid cell size. The topographic correction amounts up to tens of Eötvös for the Tzz. It becomes higher over the maximum topographic elevations (e.g., the Main Andes) and lower over the topographic depressions such as the Chilean trench.

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Chapter 20  Crustal structure delineated by satellite gravimetric models

2.2 ­2-D gravity forward model With the purpose of analyzing the continuity of the Malvinas and Colorado crustal faults that affect the passive margin area under the sedimentary cover over the continent, a 2D gravimetric model was made, which is orthogonal to the traces of Malvinas and Colorado faults, respectively. A forward modeling of the gravimetric section A-B (see Fig.  1 for location of the profile) was performed using bathymetric, gravimetric, magnetic, and multichannel seismic data grids acquired by COPLA (Arecco et al., 2016). This information includes deep seismic data, which allowed determining different crustal discontinuities. The 2-D forward model considers simple crustal blocks based on seismic data, Moho depths obtained from inversion of gravity anomalies and geological information. These constraints consisted in sea floor depths, a collection of reflection depths, location of crystalline basement, SDR (seaward dipping reflectors), and HVLC (high-velocity lower-crust), that adjusted the deep seismic data with satisfactory precision. We assigned different densities for seawater (1027 kg m−3), upper crust (2670 kg m−3), lower crust (2920 kg m−3), and mantle (3300 kg m−3), following Introcaso (2003). Densities for sediments were obtained from compressional waves velocities, which were converted to densities using Gardner’s relation according to Brocher (2005). For interlayered igneous rocks, densities were slightly modified from Schnabel et al. (2008). For inversion, we used the interactive GravModeler software, which allows fitting the calculated gravitational anomaly to the observed gravitational anomaly by changing the geometry of the structures and varying densities of the source blocks. The algorithm used to calculate the response on GravModeler is based on Talwani and Ewing (1960) algorithm implemented by Geotools Corporation (http://gravmodeler.software.informer.com/). The parameters used for the 2D model are indicated in Table 1, and the final model in Fig. 4.

2.3 ­Depth to the crust-mantle interface (Moho) In order to present the difference between the isostatic and the inverted Moho, we used the AiryHeiskanen concept and the Moho model (Fig. 5) obtained by the nonlinear gravimetric inversion from Uieda and Barbosa (2017). Table 1  Density parameters used in the forward model of profile A-B (Fig. 4) No.

Unit

Density (kg m−3)

1 2 3 4 5 6 7 8 9

Seawater Sediment Mantle Upper continental crust Lower continental crust Upper transitional crust Lower oceanic crust Lower transitional continental crust High-velocity lower crust (HVLC)

1027 2400 3300 2700 2880 2780 2830 2870 3030

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MiliGal

40 20 0 –20 –40 0

MF

SW

CF

NE 1.027 2.40

5

km

10

2.70

2.70

2.78

15 20

2.88

2.83 2.88

25

3.03

30

0

3.30 160 km

Water Sediments Mantle High-velocity Lower crust

Upper continental crust Lower continental crust CF Colorado mega fault MF Malvinas mega fault

Upper transitional crust Lower transitional crust Observed gravity Calculated gravity

FIG. 4 Forward gravimetric model of section A-B through the northern edge of the Malvinas plateau (see Fig. 1). The geometry of the blocks and the densities were extracted from the data collected by COPLA, constrained by seismic data (Arecco et al., 2016).

2.3.1 ­Moho from gravity inversion

The Moho depths model, calculated by Uieda and Barbosa (2017), is based on satellite gravity and seismological data and combines the highly efficient Bott’s method with smoothness regularization and a discretization of the anomalous Moho into tesseroids (spherical prisms). The inversion model was controlled by three hyperparameters: the regularization parameter estimated by the method of holdout crossvalidation, the anomalous Moho density contrast and the reference Moho depth; the last two estimated using the knowledge of the Moho depth at control points (for more details, see Uieda and Barbosa, 2017). In particular, Moho depths are estimated for the entire South American continent using the corrected gravity anomaly and seismological data sets (Assumpção et al., 2013), which were used for the validation of the inversion. These authors obtained the gravity anomaly from the Earth gravity field model GOCO5S (MayerGuerr, 2015) by removing: (I) the normal gravity, (II) the gravitational effect of the topography and basins infills. Corrections were calculated by modeling the ETOPO1 digital terrain model (Amante and Eakins, 2008) and the CRUST1.0 model (Laske et al., 2013), using tesseroids and a standard density of 2670 kg m−3 for continents and −1630 kg m−3 for the oceans (for more details see Uieda and Barbosa, 2017).

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Chapter 20  Crustal structure delineated by satellite gravimetric models

Fig.  5 presents the Moho model obtained by Uieda and Barbosa (2017) with a density contrast of 400 kg m−3 and a reference level of 35 km.

2.3.2 ­Isostatic Moho

Isostatic studies have previously been performed in the region by Wienecke et al. (2007) and Tassara et al. (2007), based on terrestrial gravity data and GRACE satellite data, respectively. In the present work, we analyze the differences between the ripples of an isostatically compensated Moho and a Moho obtained by gravimetric inversion from the observed data.

FIG. 5 Moho obtained by nonlinear gravimetric inversion using data of satellite gravity and seismological data (Uieda and Barbosa, 2017). The main crustal structures are interpreted by means of the vertical gravity gradient and geological background.

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The Airy-Heiskanen hypothesis considers that the mass excess of the topography is balanced by replacing mantle material by relatively lower density material in the form of a crustal root. To calculate the isostatic Moho, we used the follow parameters: a topographic density of 2670 kg m−3, a water density of 1030 kg m−3, a density contrast between crust and mantle of 400 kg m−3, and a normal crustal thickness of 35 km according to seismologic studies (Assumpção et al., 2013). We averaged the ETOPO1 model using a grid with cell size of 10 × 10 km2 without low-pass prefiltering.

2.3.3 ­Difference of both Mohos

We calculated the difference between both Mohos (Isostatic Moho-Moho obtained from gravimetric inversion) to determine the compensation degree of the study area (see Fig. 6).

3 ­Results The topography-corrected vertical gravity gradient (Tzz) from EIGEN-6C4 and GOCE GO_CONS_ GCF_2_DIR_R5 models (Figs.  2 and 3) presents the greatest spatial resolution available to date. However, this model may be influenced by topography particularly in those regions with a lack of terrestrial data, similar to EGM2008 (Pavlis et al., 2012). Main differences in EIGEN6C4 with respect to the GOCE GO_CONS_GCF_2_DIR_R5 model, when compared at the same degree order, arise from the terrestrial data that are included in the first model. Some anomalies present in EIGEN6C4 (Fig. 2) are not detectable in the Tzz map from GOCE GO_CONS_GCF_2_DIR_R5 model (Fig. 3) due to its lower spatial resolution. Nevertheless, both Tzz range between approximate maxima and minima respectively, with differences minor than ±10 Eötvos at extreme values. The effect of the oceanic Nazca plate reaches its maximum values over the Tzz signal at the outer rise area and its minima at the Central Andean axis. The inverted Moho displays a range of depths from 69 to 7 km (Fig. 5), being the thickest crustal roots in the Central Andes region. The inverted Moho compensates the topographic load up to approximately 35°S (Introcaso et al., 1992), where the topography mass decreases substantially as well as the Moho depth. The sea floor exhibits the shallowest Moho depths as expected. The Moho depths through the foreland zone vary from −60 to −35 km on the W-E direction all through the foreland region from the Southern Central to the Patagonian Andes region. At the Chilean-Pampean flat subduction zone latitudes, beneath the Precordillera region the Moho presents an average depth of −55 km that falls up to −37 km to the Sierras Pampeanas in the east, and even to −35 to −30 km along the Rio de La Plata Craton region. To the south, the Moho is detected at −40 km depth in the Deseado Massif in southern Patagonia. Both isostatic and gravity inverted Mohos have a similar pattern, with a few kilometers of difference with values between −73 and −5 km. North of 38°S, the Andes are isostatically compensated, although south of this latitude dominates and overcompensation stage (e.g., Introcaso et  al., 1992; Alvarez et al., 2012; Alvarez et al., 2014). Differences between the isostatic Moho and the Moho estimated by the gravimetric inversion range from −12 km to +24 km, with the Andes region slightly compensated from the Altiplano to 38°S. To the east, over the Precordillera region, Moho differences become positive (under compensated) (Fig. 6). This positive response is interpreted by a lack of roots beneath the Precordillera due to a high crustal strength and potential dynamic sustainability caused by the Chilean-Pampean flat subduction zone (Alvarez et al., 2012, 2014; Gimenez et al., 2000, 2009; Introcaso et al., 1992 for a detailed analysis).

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FIG. 6 Difference between the hydrostatic Moho and inverted Moho calculated by nonlinear gravimetric inversion (Uieda and Barbosa, 2017). The main crustal structures are interpreted from the vertical gravity gradient and geological background.

4 ­Discussion Combined EIGEN-6C4 model and satellite-only GOCE data, both corrected by the topographic effect, allow evaluating the crustal structure beneath the Southern Central and Patagonian Andes, adjacent foreland region, and even Atlantic passive margin area that was discussed in the last years, mainly sustained on geological basis. Based on our analysis, some lineaments, interpreted as crustal structures,

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associated with strong linear gravity gradients can be directly associated with previously identified sutures and crustal boundaries while others need to be further evaluated or redefined particularly for the Patagonian region. In the Chilean-Pampean flat subduction zone, to the east of the Precordillera region, a broad broken foreland zone is developed extending to the eastern Sierras Pampeanas up to the western border of the Rio de la Plata Craton (Fig. 1). These uplifts are represented by Neoproterozoic and Early Paleozoic rocks, which are characterized by low gradient gravity values typical of a dominant granitic composition (Baldo et al., 1996; Rapela et al., 1990). The gravity signal indicates that this eastern mountain system is sustained by intermediate Moho depths (approx. 40 km), which are consistent with previous determinations (Gans et al., 2011; Gimenez et al., 2000; Miranda and Introcaso, 1996). Additionally, different authors have found that the eastern Sierras Pampeanas are undercompensated (Introcaso et al., 1992; Introcaso, 2000) for wavelengths in the order of range topography (<50 km), based on terrestrial gravity data. Neoproterozoic and Early Paleozoic crystalline basement of the Sierras Pampeanas is clearly identified through the Tzz map from EIGEN-6C4 model (with higher resolution) (Fig. 2). The Tzz map calculated from GOCE model (Fig. 3) presents lower spatial resolution, impeding such degree of detail since the topographic signal is in the same order of magnitude. To the east of the Sierras Pampeanas, basement outcrops of the Rio de La Plata Craton (RPC) extend with an approximate area of 20,000 km2. The RPC is characterized by isolated outcrops of 2.2 and 1.7 Ga whose ages coincide with borehole data that allowed defining its westernmost extent (Rapela et  al., 2011). The contact between the Sierras Pampeanas basement and the RPC is clearly delineated by means of the gravity anomaly through the Tzz (Figs. 2 and 3) (Alvarez et al., 2012) and the relative Moho depths that become contrastingly shallower in the east (Fig. 4) (Uieda and Barbosa, 2017). South of the Chilean-Pampean flat subduction zone, in the Payenia region and the San Rafael block corresponding to the retro-arc region of the Mendoza Province in Argentina (Cingolani, 2016), the Moho becomes contrastingly deeper, with the isostatic residual Moho (or the difference between the isostatic and inverted Moho) closer-to-zero in the south and under-compensated in the north. To the southeast, over the passive margin area, the Salado basin presents a positive Tzz signal, indicating that this aulacogenic basin presents a significant attenuated crust, whose positive effect on the gravimetric signal is masking the effect of the sedimentary infill. At these latitudes, in northern Patagonia over the Andean region, determined Tzz (Fig. 3) and Moho depths (Fig.  4) indicate the presence of oblique to the margin NW crustal structures. In particular, an important interruption of the gradient signal occurs in coincidence with one of them, the Bio-Bio Aluminé-Lanalhue lineament along the retro-arc and fore-arc zones (Garcia Morabito et  al., 2003; Rosenau et al., 2006). This segmentation in the fore-arc is interpreted as reflecting the southern edge of the proposed Chilenia terrane, whose origin is more likely related to a parautochthonous origin detaching from the Gondwana margin and the western Cuyania terrane in Early Paleozoic times (Hackney et al., 2006; Krawczyk et al., 2006; Ramos et al., 2011; Sick et al., 2006; Heredia et al., 2018; among others). To the south through the fore-arc and arc regions, the recently proposed Chaitenia microcontinent (Herve et al., 2017) is characterized by a Tzz-positive response due to a higher density, which is compatible with its proposed island arc character and shallower Moho also characteristic of a narrower oceanic-crust accreted to the continental margin, as inferred, in Devonian times. These Paleozoic rocks are intruded by the Cretaceous to Cenozoic Patagonian Batholith that runs between 48° and 55°S, for >2000 km through the Andean axis. This feature is suddenly truncated in coincidence with the Magallanes-Fagnano fault system at 53°S (MFFS), represented by high Tzz values that separate the South American and Scotia plates (Dalziel et al., 1974; Ghiglione and Ramos, 2005; Klepeis, 1994;

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Chapter 20  Crustal structure delineated by satellite gravimetric models

Lodolo et al., 2002; Olivero, 1998; Ramos, 2008). The inverted Moho depths across this plate boundary indicate that the crust on the Patagonian Andes to the north (Fig. 4) is considerably thinner (−25 km) than to the south (−35 km). To the foreland zone and to the north, the transversal-to-the Andes Huincul high in North Patagonia extra-Andean region, interpreted as an inverted transversal rift system, most likely controlled by a crustal structure bordering the North Patagonian massif, is evidenced by a positive response in the vertical gravity gradient maps (Figs. 3 and 4). This structure could indicate the northern boundary of the North Patagonian microcontinent interpreted by Ramos (1984), as a parautochthonous block that was accreted to Gondwana in Early Permian times. However, the North Patagonian Massif (NPM) presents a more complex internal structure than previously assumed. This cratonic block is characterized by a minimum of Tzz (Fig. 3), which corresponds to the Cañadón Asfalto basin infill (Lince Klinger et al., 2011) that is affected by a SE-oriented crustal structure that separates two domains, northern and southern NPM. This strong gradient coincides with the inland projection of the Malvinas Fault Zone (MFZ) in the Atlantic platform to the east (Fig. 3), implying that it most likely constitutes a NW crustal structure that sections the foreland region at the NPM controlling the development of the Cañadón Asfalto Basin and continuing through the Atlantic platform. The nature of this regional structure needs to be analyzed in terms of geological evidence to interpret its tectonic significance since it cannot be directly correlated to a known proposed structure. However, one possibility is that this internal segmentation of the NPM could be related to a Pampean segmentation proposed in recent studies (see Heredia et al., 2018 for a discussion). More intriguing is its potential connection to structures segmenting the fore-arc zone as the gravity signal suggests, since the Lanalhue fault zone, potentially related to the southern edge of the Chilenia terrane, could propagate into the NPM and project toward the Atlantic passive margin. A cross-section parallel to the Argentinean continental platform (see Fig. 1), by means of a gravimetric section using COPLA data (Arecco et al., 2016), is performed in order to analyze the geometry of this crustal structure that is segmenting northern Patagonia. In the forward gravimetric model (Fig. 4), the MFZ and CFZ are depicted by positive changes in the gravity signal adjusted by the presence of crustal faults with significant displacement. To the south over the Argentinean continental platform, Tzz and Moho maps (Figs. 2, 3, and 5) signal another crustal discontinuity south of the Malvinas Fault Zone, following a strong gradient bordering the northern Deseado Massif over the continent. This structure could correspond locally to the NW-oriented suture proposed by Pankhurst et al. (2006) that delimitates South and North Patagonia and controls the development of the Neocomian rift depocenters in the western San Jorge Gulf Basin and its potential expression through the Atlantic continental platform where it would acquire a WNW orientation.

5 ­Conclusions Combined EIGEN-6C4 model and satellite-only GOCE data, both corrected by the topographic effect, allow analyzing the crustal structure of the transitional zone between the Southern Central and Patagonian Andes and adjacent foreland zone. We calculated gravity anomalies and vertical gravity gradients that allow inferring Moho depths and crustal structures that segment the basement. These data signaled important contrasts in crustal thickness, general anomaly patterns (e.g., roughnesssmoothness), and linear boundaries that characterize the main proposed terrains, parauchthoctonous

­ References

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­ icrocontinents, and cratons that form the basement of the southern Andes. Even though roughly the m obtained patterns follow previous proposals for the Southern Central Andes and adjacent foreland zone, limiting with high precision in particular the western Río de La Plata craton and Pampia-CuyaniaChilenia microcontinents, an apparent higher complexity appears in Patagonia respect to previous models. In particular, a NW structure segmenting the North Patagonian Massif extending into the Atlantic platform and fore-arc region needs to be addressed in relation to the southern extent of the Chilenia microcontinent and the northern proposed suture of Patagonia. Additionally, The Famatinian suture that separates North and South Patagonia through the northern Deseado Massif shows a connection to the northern Malvinas plateau, which needs to be further verified. Over the Patagonia Andes, a strong segmentation between Chaitenia oceanic terrane, northern Magallanes basement and Scotia plate basement by means of oblique to the margin gradients needs also to be verified and interpreted tectonically.

­Acknowledgments The authors specially acknowledge Drs Sacek and Tocho for their constructive reviews. We also acknowledge Dr. L. Uieda, Dr. J. Janak, Dr. M. Sprlak, and Dr. Prof. C. Braitenberg for the use of their software. The authors would like to thank to CONICET, COPLA, and the Ministerio de Ciencia y Técnica–Agencia de Promoción Científica y Tecnológica, PICT 2014-1697 for financial support.

­References Alvarez, O., Gimenez, M., Braitenberg, C., Folguera, A., 2012. GOCE satellite derived gravity and gravity gradient corrected for topographic effect in the South Central Andes region. Geophys. J. Int. 190, 941–959. https://doi. org/10.1111/j.1365-246X.2012.05556.x. Alvarez, O., Gimenez, M., Braitenberg, C., 2013. Nueva metodologı́a para el cÁlculo del efecto topogrÁfico para la corrección de datos satelitales. Rev. Asoc. Geol. Argent. 70, 499–506. Alvarez, O., Nacif, S., Gimenez, M., Folguera, A., Braitenberg, C., 2014. GOCE derived vertical gravity gradient delineates great earthquake rupture zones along the Chilean margin. Tectonophysics 622, 198–215. https://doi. org/10.1016/j.tecto.2014.03.011. Alvarez, O., Gimenez, M., Folguera, A., Spagnotto, S., Bustos, E., Baez, W., Braitenberg, C., 2015. New evidence about the subduction of the Copiapó ridge beneath South America, and its connection with the ChileanPampean flat slab, tracked by satellite GOCE and EGM2008 models. J. Geodyn. 91, 65–88. https://doi. org/10.1016/j.jog.2015.08.002. Amante, C., Eakins, B., 2008. 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis (ETOPO1). NOAA, National Geophysical Data Center, Boulder, CO. Arecco, M.A., Ruiz, F., Pizarro, G., Gimenez, M., Martı́nez, P., Ramos, V.A., 2016. Gravimetric determination of the continental-oceanic boundary of the Argentine continental margin (from 36° S to 50° S). Geophys. J. Int. 204, 366–385. https://doi.org/10.1093/gji/ggv433. Assumpção, M., Feng, M., Tassara, A., JuliÁ, J., 2013. Models of crustal thickness for South America from seismic refraction, receiver functions and surface wave tomography. Tectonophysics 609, 82–96. https://doi. org/10.1016/j.tecto.2012.11.014. Baldo, E.G., Demange, M., Martino, R.D., 1996. Evolution of the Sierras de Córdoba, Argentina. Tectonophysics 267 (1–4), 121–142. https://doi.org/10.1016/s0040-1951(96)00092-3. Bassin, C., Laske, G., Masters, G., 2000. The current limits of resolution for surface wave tomography in North America. Eos Trans. AGU 81 (48). Fall Meet. Suppl., Abstract S12A–03.

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